1. Field of the Invention
The present invention relates to a novel fluoroelastomer having excellent processability and a vulcanized rubber obtained therefrom. More particularly, the present invention is concerned with a fluoroelastomer comprising vinylidene fluoride units and hexafluoropropylene units, optionally together with tetrafluoroethylene units, in specific proportions and having a relatively low limiting viscosity number and a specific molecular weight distribution, and a vulcanized rubber obtained therefrom. The fluoroelastomer has excellent processability during the vulcanization-molding procedure, particularly excellent roll processability, mold release characteristics and metal-adhesion properties, and a vulcanized rubber obtained therefrom has extremely excellent mechanical strength and compression set resistance.
2. Discussion Of Related Art
In general, fluoroelastomers have excellent resistance to heat, solvent and chemicals and, therefore, have been useful in the manufacture of sealant materials which are subject to being employed under stringent conditions. Examples of such materials are O-rings, oil seals, packings and gaskets; diaphragm materials and the like. In recent years, mechanical parts have more frequently become smaller in size and sophisticated in shape and, accordingly, fluoroelastomers are now required to have improved processability, mechanical strength, compression set resistance and the like.
Heretofore, various compositions have been proposed with respect to a fluoroelastomer. For example, a fluoroelastomer was proposed, which is obtained by copolymerizing 60 to 15 % by weight of vinylidene fluoride (hereinafter often referred to as "VdF") with 40 to 85 % by weight of hexafluoropropylene (hereinafter often referred to as "HFP") (see Japanese Patent Application Publication Specification No. 33-7394). Another type of fluoroelastomer is disclosed in U.S. Patent No. 2,968,649, which contains VdF, HFP and 3 to 35% by weight of tetrafluoroethylene (hereinafter often referred to as "TFE") units and wherein the total amount of VdF units and HFP units is 97 to 65 % by weight and the weight ratio of VdF units to HFP units is in the range of from 2.33:1 to 0.667:1. Still another type of fluoroelastomer is disclosed in Japanese Patent Application Publication Specification No. 48-18957, which contains VdF, HFP and 10 to 30 % by weight of TFE units and wherein the total amount of VdF units and HFP units is 90 to 70 % by weight and the weight ratio of VdF units to HFP units is in the range of from 1.6:1.0 to 4.0:1.0. Still another type of fluoroelastomer is disclosed in U.S. Patent No. 4,123,603, which comprises 57 to 61 % by weight of VdF units, 27 to 31 % by weight of HFP units and 10 to 14 % by weight of TFE units.
Fluoroelastomers are generally subjected to a primary vulcanization using a mold and to a secondary vulcanization in an oven to obtain a shaped article. However, conventional fluoroelastomers have drawbacks in that the mold release characteristics are likely to become poor after the primary vulcanization and, therefore, portions of the fluoroelastomer are likely to be torn off and left on the surface of the mold, leading to an increase in the ratio of defectives. On the other hand, fluoroelastomers are required to have good metal-adhesion properties during the vulcanization-molding. That is, when a shaped, composite rubber article comprising a vulcanized fluoroelastomer and a metal component unified therewith is intended to be produced, the fluoroelastomer is required to firmly adhere to the metal component by means of the vulcanization-molding treatment. Therefore, fluoroelastomers having not only good mold release characteristics but also good metal-adhesion properties within the vulcanization-molding procedure, even though the two are incompatible with each other, are strongly desired.
In vulcanizing a large quantity of fluoroelastomer by kneading, for example, by the use of a Banbury mixer, fluoroelastomers having a relatively low molecular weight are generally used. However, this is disadvantageous in that the resultant vulcanized, shaped rubber articles are likely to have poor mechanical strength and compression set resistance. Further, there is also a problem in that such low molecular weight fluoroelastomers have poor mold release characteristics and, therefore, portions of fluoroelastomer are likely to be torn off and left on the surface of a mold. For overcoming the above-mentioned problems, it was proposed to add to a fluoroelastomer a vulcanization accelerator, such as a phosphonate, before kneading (see Japanese Patent Application Laid-Open Specification No. 62-54750). However, the above-mentioned problems have not been satisfactorily overcome by simply adding a vulcanization accelerator. Up to the present time, no method has been proposed which is capable of satisfactorily solving the above-mentioned problems.
On the other hand, with respect to the production of a two-component fluoroelastomer consisting of VdF units and HFP units, various techniques using emulsion polymerization are known, which include those disclosed in Japanese Patent Application Publication Specification No. 36-5893 and U.S. Pat. Nos. 3,051,677, 3,056,767, 3,069,401 and No. 3,194,796.
Moreover, proposals have been made in order for a fluoroelastomer to have a specific composition (see, for example, U.S. Pat. No. 4,027,086). Also, a method has been proposed for producing a blend of a fluoroelastomer and a fluororesin, which consists in first obtaining a fluoroelastomer by emulsion polymerization and subsequently obtaining a fluororesin by emulsion polymerization to thereby obtain a blend thereof (see, for example, U.S. Pat. No. 3,929,934).
On the other hand, various proposals have been made concerning the regulation of the molecular weight distribution of a fluoroelastomer. For example, there have been proposed a method for producing a copolymer having a bimodal molecular weight distribution by a two-step emulsion polymerization process (continuous polymerization), wherein polymerization is first effected for forming a high molecular weight copolymer and subsequently for forming a low molecular weight copolymer (see U.S. Pat. Nos. 3,839,305 and 3,845,024); a method for producing a copolymer or terpolymer having a low Mooney viscosity and excellent processability, in which suspension polymerization is performed (see U.S. Pat. No. 3,801,552; a method for regulating the molecular weight distribution of a fluoroelastomer by the use of a chain transfer agent (see Japanese Patent Application Publication Specification No. 52-46998); a method for producing a fluoroelastomer having a predetermined molecular weight distribution, in which suspension polymerization is performed while portion-wise adding a polymerization catalyst (see Japanese Patent Application Laid-Open Specification No. 62-59611); a method for producing a fluoroelastomer having a bimodal molecular weight distribution, in which suspension polymerization is performed under a relatively high pressure (see European Patent Application Publication No. 0186180 A2); and a method for producing a fluoroelastomer having a predetermined molecular weight distribution, in which suspension polymerization is performed under a relatively low pressure (see Japanese Patent Application Laid-Open Specification No. 62-112611).
Moreover, the following two-step emulsion polymerization was proposed. In U.S. Pat. No. 4,141,874, it was proposed to employ a method in which a water-soluble radical initiator is first used and an oil-soluble radical initiator is subsequently used. Further, in U.S. Pat. No. 3,801,552, it was proposed to employ a method for producing a fluoroelastomer, which comprises performing polymerization under ionizing radiation so as to obtain a fluoroelastomer having a low Mooney viscosity and excellent processability.
As described above, a wide variety of fluoroelastomers are known, which can be produced by a number of methods. However, a fluoroelastomer which is satisfactory in all of the desired properties, such as melt flowability, moldability, metal mold-caused staining resistance, mold release characteristics, metal-adhesion properties and capability of providing a vulcanized, shaped rubber article having excellent compression set resistance and mechanical strength, is not known. In fact, all the prior art fluoroelastomers do not satisfy simultaneously both the apparently incompatible different properties, that is, excellent processability and capability of providing a vulcanized, shaped rubber article having excellent mechanical properties.